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United States Patent |
5,026,616
|
Schumm, Jr.
|
June 25, 1991
|
Electroless plated plastic covers for galvanic cells
Abstract
A plastic leak resistant and corrosion resistant end cover and contact for
a battery having an electroless plated layer of nickel, tin or an alloy of
nickel and tin on the surfaces thereof so that when in place in a battery
it forms a conductive path for carrying current from inside the battery to
the outside. A battery employing at least one of the afore described end
cover.
Inventors:
|
Schumm, Jr.; Brooke (Bay Village, OH)
|
Assignee:
|
Eveready Battery Company, Inc. (St. Louis, MO)
|
Appl. No.:
|
436607 |
Filed:
|
November 15, 1989 |
Current U.S. Class: |
429/168; 429/170; 429/177 |
Intern'l Class: |
H01M 002/02 |
Field of Search: |
429/168,170,177
428/461,462
|
References Cited
U.S. Patent Documents
2532283 | Dec., 1950 | Brenner et al. | 204/48.
|
3591352 | Jul., 1971 | Kennedy et al. | 428/462.
|
3795545 | Mar., 1974 | Kamai | 429/168.
|
3849172 | Nov., 1974 | Chin et al. | 428/462.
|
4732807 | Mar., 1988 | Maeda et al. | 428/462.
|
Primary Examiner: Kalafut; Stephen J.
Attorney, Agent or Firm: Welsh; Robert W.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. An electrical battery comprising:
a housing having an elongated shell and first and second open ends for
containing the components of a voltaic cell;
a first metal plated plastic end cover for closing off the first end of
said housing and for forming a first electrical contact for said battery;
and
a second metal plated plastic end cover for closing off the second end of
said housing and for forming a second electrical contact for said battery.
2. An electrical battery as set forth in claim 1 wherein the plastic used
to form the end covers is compatible with the components making up the
voltaic cell.
3. An electrical battery as set forth in claim 1 wherein the plastic end
cover is formed from an acrylontrile-butadiene-styrene engineering
plastic.
4. An electrical battery as set forth in claim 1 wherein the plastic end
cover is formed from polypropylene.
5. An electrical battery as set forth in claim 1 wherein the plastic end
cover is formed from poly(phenyleneoxide).
6. An electrical battery as set forth in claim 1 wherein the plastic end
cover is formed from polysulfone thermoplastics.
7. An electrical battery as set forth in claim 1 wherein the plastic end
cover is coated with nickel.
8. An electrical battery as set forth in claim 1 wherein the plastic end
cover is coated with tin.
9. An electrical battery as set forth in claim 1 wherein the plastic end
cover has a coated metal pattern forming an electrically conductive path
from the inside of said battery to the outside.
10. An electrical battery as set forth in claim 1 wherein the plastic end
covers have an electroless plated layer of a metal selected from the group
consisting of nickel, tin and alloys thereof.
11. An electric battery comprising:
a shell having first and second open ends for containing the several
components making up a voltaic cell;
an anode disposed in said shell;
a cathode disposed in said shell;
an electrolyte for assisting in the electrochemical reaction contained in
said shell in operating relation with said anode and said cathode;
a first end cover for closing the first end of said shell and for forming a
first electrical contact for said battery;
a second end cover for closing the second end of said shell and for forming
a second electrical contact for said battery; and
wherein at least one of said first end cover and second end cover is made
of plastic and has a conductive coating thereon completing a conductive
path from one side of said cover to the opposite side of said cover.
Description
BACKGROUND OF THE INVENTION
The batteries used in conventional home appliances or flashlights usually
have plated metal end covers which form the positive and negative contact
for the battery. The battery industry has used all-metallic covers on
galvanic cells for many years. These covers perform three functions.
First, the cover normally provides an electrical contact point for the
cell. Second, the internal battery components are protected from external
hazards. And third, exudate from the cell is contained within the battery.
The covers are usually made of steel which has been plated with a metal
such as nickel or tin. Although these covers have been used successfully
in commercial products, there has always been a problem with corrosion of
the covers' base steel. This type of problem is particularly noticeable
when batteries have been completely discharged and then stored for an
extended period of time. This situation can occur when a flashlight's
batteries have been run down through continuous operation and then left in
the flashlight for several months.
Conventional covers for carbon-zinc batteries have traditionally had a
problem with corrosion whenever the cell's exudate was able to contact the
cover or when the cell was exposed to humid conditions for a long period
of time. Corrosion due to leakage is particularly noticeable after the
cell has been on a long continuous discharge. The covers are usually
produced by forming and punching the part from preplated steel strips.
This material has micropores in the protective plate and develops stress
cracks in the punching operation. When the cell's acidic electrolyte
penetrates these micropores and stress cracks, corrosion of the base metal
can occur. This problem detracts from the appearance of the battery and
can proceed to the point that the battery will not function.
SUMMARY OF THE INVENTION
In accordance with the present invention, improved covers are provided for
a conventional battery through the use of electroless plated plastic
material. Electroless plating techniques are used to apply a thin layer of
metal, such as nickel or tin, to the plastic top and bottom covers of a
dry cell battery. Using plastic instead of steel as the base material for
the cover eliminates the possibility of cover perforation and the rust
color that is caused by the corrosion of the iron in the base steel. Even
if the relatively noble metal is too thin to prevent some electrolyte
contact with the base part, the plastic cannot corrode. Scratches or
pinholes through the protective metal layer may be visually distracting,
but they will not lead to corrosion, perforation, and failure of the
cover. Electroless plating techniques are employed to produce a thin
coating of nickel or tin on both sides of the cover and on the circular
edge of the cover to provide a continuous electrically conductive path
from the interior of the dry cell battery to the external contact surface.
An improved corrosion resistant electric battery is also provided having a
shell with first and second open ends for containing the several
components making up a galvanic cell. For example, an anode, cathode and
electrolyte material are contained within the shell. A first plastic end
cover having an electrolessly plated metallic coating thereon is provided
for closing one end of the shell and for forming a first electrical
contact for the battery. A second plastic end cover having an
electrolessly plated metal coating thereon is used to close a second
opening in the shell and for providing a second contact for the battery.
Through the use of the electrolessly plated plastic covers a substantially
improved corrosion resistant battery is provided.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view partially in section of the containment means
used with a galvanic cell; and
FIG. 2 is a sectional view of a galvanic cell incorporating the electroless
plated end covers of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In FIG. 1 there is illustrated the containment means for a conventional dry
cell battery. The containment means is indicated generally by the number
10 and consists of end covers 11 and 13 and an electrically nonconductive
jacket 15. The nonconductive jacket is in sealing engagement with and
situated between the end covers 11 and 13 such that the covers are
electrically insulated from one another. In the preferred embodiment
illustrated in FIG. 1, both end covers for the battery 10 are made up of a
plastic inner layer 17 and a conductive coating of electroless plated
metal 19. While the preferred embodiment uses a plastic cover at each end
of the battery, it is certainly within the scope of the present invention
to use only one metal-coated plastic cover for the battery and have the
other or remaining cover a conventional plated steel cover.
The plastic covers can be made by any well-known plastic forming or cutting
technique. For example, the covers could be extruded, injection molded or
preferably hot stamped from strip or sheet material. In the hot stamping
process, the sheet of plastic can be cut to size and any contact or
reinforcing shape added to the cover at the same time. For example, one
cover of a battery usually has a raised central button for forming a
contact while the other cover usually has some type of pattern such as
circular corrugations to reinforce the otherwise flat surface.
The size of the cover is dependent on the size of the standard cell for
which it is being prepared. The size, for example and not by way of
limitation, can run anywhere from the AAA to a D size cell. The thickness
of the cover can vary. It should be strong enough to safely contain gas
pressure generated in the cell and to protect and contain the components
of the cell while in operation and during shipping.
Many different plastic or polymeric materials can be used to form the
covers. Borg-Warner, General Electric, Monsanto and Uniroyal all
manufacture thermoplastic resins suitable for use in electroless plating.
While many different plastic materials can be used functionally to satisfy
the use of the present invention, there are preferred materials which,
when plated, produce a cover with a superior overall appearance. The
following materials have been determined to produce battery covers with
outstanding physical appearance. The preferred plastic materials are ABS
(acrylonitrile-butadiene-styrene) engineering thermoplastics, polysulfone
thermoplastics, polypropylene and modified poly(phenyleneoxide).
Plastic end cover blanks are preferably cut and shaped and then subjected
to a surface treatment using chromic acid, sulfuric acid and water. The
etching process converts the normally hydrophobic surface to a hydrophilic
surface and roughens this surface to a small extent to increase the
adhesion of the subsequently applied electroless plated metal. After the
etching process is completed, the surface is neutralized using a mild
basic or acidic reducing agent. Neutralization removes a large amount of
the hexavalent chromium from the surface. Hexavalent chromium shortens the
life of the catalyst, and trace amounts can completely inhibit electroless
nickel deposition.
The electroless plating of metal upon non-conductive surfaces, and
particularly plastic surfaces, is well-known in the art. The electroless
plating of nickel was the subject of U. S. Pat. No. 2,532,283 issued Dec.
5, 1950, to A. Brenner and G. Riddell and assigned to the United States
government. The patentees were researchers at the National Bureau of
Standards and found that nickel could be electrolessly plated by the
autocatalytic chemical reduction of aqueous metal ions. The metal ions are
reduced to metal by the action of the chemical reducing agent or electron
donor which supplies electrons to the positively charged metal ions to
reduce the ion to the elemental state. The components of the electroless
bath include an aqueous solution of metal ions, a catalyst, reducing
agent(s), complexing agent(s), and bath stabilizers operating at a
specific metal ion concentration, temperature and pH.
As discussed above, it is preferred that the plastic surface be given an
initial etch to prepare the surface for the electroless deposition. The
surface can also be treated with a catalyst such as tin chloride or
paladium chloride to provide active sites on the surface. If a
well-cleaned and prepared plastic surface is immersed in the electroless
plating bath, any deposition of metal on the surface would catalyze the
continual plating process until the desired thickness of plate is obtained
or the bath is depleted.
A detailed description of the background and techniques for electroless
plating is presented in Kirk-Othmer Encyclopedia of Chemical Technology,
Third Edition, Volume 8, at pages 738-750. Another source of information
on electroless plating is Metal Finishing-Guidebook and Directory Issue,
1989, published by Metal and Plastic Publications Inc., 3 University
Plaza, Hackensack, NJ, pages 387-401. The contents of both of these
reference works are incorporated herein by reference. The following are
compositions of representative electroless nickel plating baths:
A low-temperature bath consists of the following components:
______________________________________
Nickel sulfate 30 g/L
Sodium hypophosphite 30 g/L
Sodium pyrophosphate 60 g/L
Triethanolamine 100 ml/L
pH 10.0
Temperature 30 to 35.degree. C.
(86 to 95.degree. F.)
______________________________________
An example of a high-temperature, alkaline, electroless nickel-phosphorus
bath is
______________________________________
Nickel sulfate 33 g/L
Sodium citrate 84 g/L
Ammonium chloride 50 g/L
Sodium hypophosphite 17 g/L
pH 9.5
Temperature 85.degree. C. (185.degree. F.)
______________________________________
The following is an example of an acid hypophosphite reduced bath:
______________________________________
Nickel sulfate 28 g/L
Sodium acetate 17 g/L
Sodium hypophosphite
24 g/L
Lead acetate 0.0015 g/L
pH 4.6
Temperature 82 to 88.degree. C.
(180 to 190.degree. F.)
______________________________________
The thickness of the electroless plate is not critical so long as it is
thick enough to carry the current produced by the cell. To increase the
thickness of the plate, the substrate can be left in the bath for longer
periods of time. The electroless bath can deposit metal at approximately
0.5 to 1 mil per hour. The part can be left in the bath until sufficient
thickness is obtained, or once the surface has been made conductive, the
part can be moved to an electroplating bath where a thicker coat or a
flash coat of metal can be applied.
Through the use of a combination of plating techniques, it is possible to
apply electroless nickel, tin, and alloys thereof to the plastic substrate
and then plate nickel or tin over the deposited substrate to the thickness
desired. To improve the appearance of the cover, the conditions of the
electroplating process can be adjusted, as is well-known in the art, to
provide a bright finish.
Referring to FIG. 2, an improved dry cell can be prepared using
electroless-plated plastic end covers at each end of the cell. The end
cover 11 has a raised contact portion 21. The end cover 13 has a
reinforcing corrugation 23 formed therein. It can be seen in FIG. 2 that
end cover 13 is in contact with the zinc cup at point 25. The electroless
metal covering 19 on the end cover 13 forms an eleotrically conductive
path from the contact point 25 where the zinc surface is contacted out and
around to the exposed surface of the end cover. In a similar manner the
inner surface of end cover 11 contacts the cathode's current collector 27
and has an electrically conductive path across the inner surface, around
the edge and onto the outer surface of the raised contact surface 21.
The cell illustrated in FIG. 2 is a conventional Leclanche cell having a
zinc cup or external anode 29, a porous separator 31, graphite powder or
carbon black and manganese dioxide moistened with ammonium chloride and
zinc chloride 33, and a carbon rod current collector 27.
Other elements illustrated in FIG. 2 are conventional to Leclanche cell
construction and include a support washer 35, a seal 37, a washer 39, and
a cup-shaped insulator (41) located at the bottom of the zinc can. The
outer jacket 15, to which the end covers 11 and 13 are tightly sealed,
completes the cell.
Several cells of the type shown in FIG. 2 were prepared and then subjected
to a rapid discharge. The cells were then placed on long-term, room
temperature storage to simulate being left in a flashlight in a discharged
condition. After several months, it was noted that the zinc electrode was
substantially corroded in all of the cells and that the electrolessly
plated end caps were still serviceable and in only minor cases showed
evidence of deterioration. It was apparent from these tests that the
electroless plated end covers were a substantial improvement in the cells.
Though the invention has been described with respect to a specific
preferred embodiment thereof, many variations and modifications will
become apparent to those skilled in the art. It is therefore the intention
that the appended claims will be interpreted as broadly as possible in
view of the prior art to include all such variations and modifications.
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